The present invention relates to instrumentation used to conduct dynamic mechanical analysis on a sample during and after a cure cycle. In particular, it relates to a fiber optic system capable of performing dynamic mechanical analysis.
It is often difficult to determine the extent of cure of thermosetting polymers during the manufacturing of polymer matrix composites. Typically, a composite is cured using a manufacturer-recommended time-temperature schedule instead of directly monitoring the resin properties as the sample cures. Once the schedule is complete, the resulting part is inspected and tested to insure that the mechanical properties are acceptable.
A number of approaches have been proposed to remove the guesswork from the cure cycle and to insure that the thermosetting polymer matrix composite is fully cured prior to use. Such approaches include measuring the refractive index of the resin; measuring the dielectric properties of the resin; or monitoring the acoustic attenuation of the material. More recently, investigators (Russell G. May et al., xe2x80x9cIn-Situ Fiber Optic Sensor for Composite Cure Monitoring Through Characterization of Resin Viscoelasticity,xe2x80x9d Proc. Of SPIE, Vol. 2948, pp 24-34, December 1996; and Russell G. May et al., xe2x80x9cMultifunctional Fiber Optic Sensor for Manufacturing of Thermoset Matrix Composite Materials,xe2x80x9d Proc. Of SPIE, Vol. 3044, pp 244-251, March 1997) discovered that coupling a fiber optic strain sensor to an actuator, will yield a miniature dynamic mechanical analysis system. By immersing the sensor in a curing thermoset resin, a time-varying excitation is applied to the actuator which causes the sensor to vibrate harmonically. A comparison of the phase of the excitation to the phase of the resulting strain as detected by the strain sensor makes it possible to derive the loss tangent of the resin which is related to the degree of cure of the resin. When the resin is completely cured, the fiber optic sensor functions as a conventional strain sensor, permitting in-service strains in the composite part to be measured.
These particular sensors have various short-comings. The piezoelectric actuators typically used for constructing the sensor assembly are brittle or fracture easily during typical composite manufacturing processes, such as hot pressing. In addition, absolute measurements of the complex modulus of the resin depends on the distance separating the vibrating actuator from any adjacent unmoving boundaries. Since this distance cannot generally be fixed in composite manufacturing, the complex modulus cannot be measured in an absolute sense. Only relative changes in modulus can be determined. Lastly, the determination of the vibration amplitude by the use of fiber optic strain gages is sensitive to optical losses that may result in the optical fiber or the optical fiber connector.
An object of the present invention is to provide a fiber optic system that uses a robust piezoelectric actuator which is capable of withstanding the application of pressure.
Another object of the present invention is to establish fixed boundary conditions for the sensor assembly.
Another object of the present invention is to render the determination of the vibration amplitude insensitive to fiber optic losses.
These and other objects were achieved by the present invention which is for a fiber optic system which comprises a laminated, pre-stressed, piezoelectric actuator; and a fiber optic strain gage having an input/output fiber. The fiber optic strain gage is attached to a surface of the laminated, pre-stressed, piezoelectric actuator. In a further embodiment of the invention, the fiber optic system comprises a high voltage amplifier and frequency generator attached to fine gauge wires that are attached to an electrode pair on the laminated, pre-stressed, piezoelectric actuator. In addition, a signal processing unit is positioned in an operable relationship to the input/output optical fiber and the fine gage wires. In yet another embodiment, a micromachined silicon housing surrounds the actuator and the fiber optic strain gage.
The fiber optic system of the present invention is useful for performing dynamic mechanical analysis on a sample. In operation, the fiber optic system is provided and a sample is exposed to the system. A stress is applied to the sample through the actuator and a change in the strain amplitude signal of the sample is measured using the signal processing unit.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.